Age-related macular degeneration (AMD) is a form of degeneration that results when the delicate photoreceptors deteriorate in a highly specialized region of the central retina called the macula. AMD is the leading cause of visual impairment and blindness for many people over age 50. The cause of AMD is not fully understood, and at present, there is no cure.
AMD is an eye disease of the macula: a tiny area in the retina that helps produce sharp, central vision required for central visual activities such as reading, sewing, and driving. A person with AMD loses this clear, central vision and in some cases, vision loss is rapid and profound. AMD is a leading cause of severe visual impairment and blindness in the United States. According to current statistics, approximately 1.5 million citizens in the United States are affected by advanced age-related macular degeneration. This number is expected to increase to 2.95 million Americans by the year 2020, according to current government statistics.
There are two forms of AMD: an atrophic form, called dry AMD, and an exudative form (eAMD), also called wet AMD. Dry AMD is the early stage of the disease; about 90% of the diagnosed cases of AMD are the dry form, but it is the wet form that results in most of the vision loss associated with the disease. The term AMD is sometimes used to refer only to the advanced form of the disease and the term Age-related Maculopathy (ARM) is used to describe the early clinical findings associated with AMD.
Dry AMD is associated with extracellular deposits called drusen (druse is the singular form of the word but is not commonly used) that form between the retinal pigment epithelium (RPE) and Bruch's membrane. Drusen are believed to result from impaired metabolism in the RPE. In a normal eye, the RPE serves a number of roles critical to healthy vision: renewal of the photoreceptor outer segments through phagocytosis, providing a blood-retinal barrier through the tight junctions between RPE cells, and the selective transport of nutrients across Bruch's membrane to the outer retina. Dry AMD usually results in a gradual loss of central vision in the macular regions associated with the drusen and loss of the RPE.
Wet AMD is also associated with the drusen deposits plus new blood vessel growth or neovascularization. Wet AMD results when fragile blood vessels grow from the choroid into the subretinal space, leaking blood and fluid, and leading to rapid loss of central vision. The growth of new vessels under the retina is called choroidal neovascularization, or CNV. Exudative AMD is a major cause of severe vision loss. Wet AMD is a major cause of severe vision loss and accounts for approximately 80% of such cases. Wet AMD often causes rapid decline in visual acuity.
The eye includes three tissue layers, or tunics as shown in the partial sagittal section of the human eye in FIG. 1. The outermost layer of the eye is fibrous tunic 10 and includes the transparent cornea (near lens 12) and the opaque white sclera. The middle, highly vascularized layer of tissue within the eye is called uveal tract 15. The innermost tunic, or layer, of the eye is retina 20, which is an extension of the central neural tissue of the brain. Retina 20 is comprised of the multi-layered neurosensory retina 22 and a tightly spaced monolayer of hexagonal-shaped cells called the RPE 25 as shown in FIG. 2. Bruch's membrane 35 is a thin, collagenous membrane separating RPE 25 from choroid 30 as shown in FIG. 2. Although RPE 25 must be in close apposition to neurosensory retina 22 for normal visual function, there is only a weak attachment between these two tissue layers. In both pathologic and surgical retinal detachments, neurosensory retina 22 is separated from RPE 25, with RPE 25 adhering to Bruch's membrane 35. In the present subject matter, it is understood that a graft to be harvested following retinal detachment typically includes RPE 25 lying on Bruch's membrane 35 and underlying choroid 30. As one looks directly into the eye, the central region responsible for the greatest visual acuity is called the macula (40), a circular region approximately 5.5 mm in diameter, as shown in FIG. 1. Macula 40 contains the highest concentration of cone photoreceptor cells that are largely responsible for central, sharp vision and color vision. In the figure, macula 40 is shown in the middle of two temporal arcing vessels (trajectories). Macula 40 is responsible for the central 15 to 20 degrees of visual angle. At the center of macula 40 is the fovea, a 1.5 mm diameter region that contains primarily cone cells.
The human eye can be described as a space-variant optical system because the detector elements within retina 20 (photoreceptors 21) vary as a function of position. Photoreceptors 21 convert light energy entering the eye into electrochemical impulses and are often referred to as rods and cones.
Improved systems and methods for addressing AMD as well as other forms of macular disease such as hereditary macular disorders, post-inflammatory diseases, post-traumatic maculopathy, and toxic maculopathy.